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This innovation offers a number of advantages in terms of reduced mass, problem of routing,
and the risk of x-ray attenuation.

Flight mirror assemblies (FMAs) of
large telescopes, such as the International X-ray Observatory (IXO), have
very stringent thermal-structural distortion
requirements. The spatial temperature
gradient requirement within a FMA
could be as small as 0.05 °C. Conventionally, heaters and thermistors are
attached to the stray light baffle (SLB),
and centralized heater controllers (i.e.,
heater controller boards located in a
large electronics box) are used. Due to
the large number of heater harnesses,
accommodating and routing them is
extremely difficult. The total harness
length/mass is very large. This innovation
uses a thermally conductive pre-collimator
to accommodate heaters and a distributed
heater controller approach. It
minimizes the harness length and mass,
and reduces the problem of routing and
accommodating them.

Heaters and thermistors are attached
to a short (4.67 cm) aluminum portion
of the pre-collimator, which is thermally
coupled to the SLB. Heaters, which have
a very small heater power density, and
thermistors are attached to the exterior
of all the mirror module walls. The major
portion (23.4 cm) of the pre-collimator
for the middle and outer modules is
made of thin, non-conductive material. It
minimizes the view factors from the FMA
and heated portion of the pre-collimator
to space. It also minimizes heat conduction
from one end of the FMA to the
other. Small and multi-channel heater
controllers, which have adjustable set
points and internal redundancy, are
used. They are mounted to the mechanical
support structure members adjacent
to each module.

The IXO FMA, which is 3.3 m in diameter,
is an example of a large telescope. If
the heater controller boards are centralized,
routing and accommodating heater
harnesses is extremely difficult. This
innovation has the following advantages.
It minimizes the length/mass of the
heater harness between the heater controllers
and heater circuits. It reduces the
problem of routing and accommodating
the harness on the FMA. It reduces the
risk of X-ray attenuation caused by the
heater harness. Its adjustable set point
capability eliminates the need for survival
heater circuits. The operating mode
heater circuits can also be used as survival
heater circuits. In the non-operating
mode, a lower set point is used.

This work was done by Michael Choi of Goddard
Space Flight Center. GSC-16380-1

This Brief includes a Technical Support Package (TSP).

An Active Heater Control Concept to Meet IXO Type Mirror Module Thermal-Structural Distortion Requirement (reference GSC-16380-1) is currently available for download from the TSP library.

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